In motor vehicles, there is a transmission interposed between an internal-combustion engine and vehicle wheels. This transmission changes wheel driving power and running speed to suit vehicle operating requirements which have a wide range of variation, thereby utilizing the performance of the internal-combustion engine to the fullest.
At present a continuously variable transmission is used to perform shift control for the purpose of changing a gear ratio by increasing or decreasing the turning radius of a belt mounted on a pulley by hydraulically increasing or decreasing the width of a groove formed between a fixed pulley member fixedly mounted on a rotating shaft and a movable pulley member which is movably mounted on the rotating shaft and which can move into contact with, and away from, this fixed pulley member.
This type of continuously variable transmission has been disclosed in Laid-Open Japanese Patent Application No. 64-44346. This transmission is designed to determine the optimum target speed from a first and a second target speed based on each of throttle opening and vehicle-speed detection signals, and to perform shift control in accordance with this optimum target speed, thus facilitating obtaining operating characteristics which suit the desires of the driver.
In the prior art shift control method, the overdrive line and full-low line are the boundary ratio lines which define therebetween the shift control range, and these lines are determined on the basis of the mechanical dimensions of the belt pulley members and other constitutive elements of the continuously variable transmission. However, since the engine speed is controlled for example to the overdrive line, the prior art shift control method has the disadvantage that when the lower limit value of the target engine speed is set at the same value as the theoretical overdrive line, if there occurs a mechanical difference or a dimensional variation of such parts as belt pulley members (sheaves), then the true overdrive line may be shifted from the theoretical line and the value of engine speed will be controlled not to the true overdrive ratio line of the continuously variable transmission but to an intermediate ratio line. That is, if the true overdrive ratio has become lower than the theoretical ratio, then the true shift control range will not be fully available.
Also the prior art shift control method has the disadvantage that even when a value of vehicle speed of an actual ratio due to the detection accuracy of a vehicle speed sensor is input into a control means, it is impossible to control to a desired overdrive ratio.
Meanwhile, another disadvantage is that when the lower-limit value of the target engine speed has been set to a much smaller ratio value than the overdrive ratio value, it takes much time to control the engine speed, hence causing the occurrence of a shift delay when controlling the engine speed from the overdrive ratio to an intermediate ratio or a full-low ratio.
Furthermore, another disadvantage is that there similarly occurs a shift delay when the engine speed is controlled from the full-low ratio to the intermediate ratio or the overdrive ratio.
The present invention has been accomplished in an attempt to obviate the aforementioned disadvantages and has an object the provision of a shift control method of a continuously variable transmission in which the limit value of the target engine speed is set to the vicinity of a ratio line determined according to machine dimensions, thereby enabling the prevention of excessive shift delay by properly controlling the engine speed in the event of dimensional variation of parts or an engine-speed signal error.
To accomplish the aforesaid object, the present invention provides a shift control method of a continuously variable transmission, which controls the shifting in order to change the gear ratio by increasing or decreasing the turning radius of a belt mounted on a pair of adjustable pulleys comprising a fixed pulley member and a movable pulley member which is movable into contact with, and away from, the fixed pulley member. The belt ratio is changed by increasing or decreasing the width of a groove provided between these two pulley members. In this shift control method, there is provided a control means for setting the target engine speed using input throttle opening and vehicle speed signals, whereby the limit value of the target engine speed is set to the vicinity of the ratio line of the shift control range as determined by constitutive elements of the transmission, but outside of the shift control range.
According to the method of the present invention, the control means receives the throttle opening and vehicle speed signals, sets the target engine speed, and sets the limit value of the target engine speed to the vicinity of the ratio line of the shift control range but outside of the shift control range, thereby avoiding excessive shift delay by properly controlling the engine speed in the event of a dimensional variation of components or an engine-speed signal error.
In the prior art shift control method, an actual engine speed error from a target engine speed is processed by proportional and integral processings to perform shift control in order to change the gear ratio. The ratio line which is a gear ratio limit value of this shift control is determined as an OVERDRIVE line or a FULL-LOW line according to machine dimensions of constitutive elements of the continuously variable transmission, the shift control being carried out in an intermediate ratio range between the OVERDRIVE line and the FULL-LOW line.
However, the actual ratio can move to the vicinity of the OVERDRIVE line or the FULL-LOW line, and when the actual engine speed is positioned on this ratio line, the target engine speed sometimes goes out of the intermediate ratio range set by the OVERDRIVE line and the FULL-LOW line, thus being positioned outside of the shift control range.
In this case, as there always takes place an error between the target engine speed and the actual engine speed, the accumulation of integral values continues, consequently resulting in an abnormal integral value and deteriorated shift control quality. For example, the actual engine speed fails to follow up a change in the target engine speed, and accordingly lowered follow-up performance results.
To obviate such a disadvantage when the ratio value has moved to the vicinity of the ratio line, a conceivable method of preventing the integral value from growing into an abnormal value is the prevention of accumulation of integral values by forbidding integral processing. However, if the integral processing is forbidden based solely and otherwise unconditionally on the ratio value, the integral processing required remains unexecuted and only the proportional processing is executed when the engine speed is changed to the intermediate ratio range side, resulting in a shift delay.
The present invention has been accomplished in an attempt to solve the problem mentioned above, and has an object to provide a shift control method of a continuously variable transmission capable of avoiding the deterioration of the follow-up performance of the target engine speed resulting from the occurrence of an abnormal integral value, and realizing a shift control method of a continuously variable transmission capable of preventing the shift delay caused by the unconditional prohibition of integral processing.
To accomplish the aforementioned object, the present invention provides a shift control method of a continuously variable transmission, characterized in that there is provided a control means for controlling shifting by subjecting an error of an actual engine speed from a target engine speed to proportional and integral processings, so that when the actual gear ratio has been changed by the control means to the vicinity of the gear ratio limit value of an intermediate gear ratio range, shift control is effected to forbid the integral processing except when specified conditions exist. Also, when the actual gear ratio is shifted from the vicinity of the gear ratio limit value to the range of the intermediate gear ratio, the integral processing is executed in order to change the gear ratio.